The long term goal of this proposal is to uncover mechanisms that underlie the disabling, painful neuropathic disease that often follows peripheral nerve injuries. In this regard, central sensitization plays a central role, and ectopic discharges from injured afferents initiate and maintain this central sensitization. Accordingly, the mechanisms that lead to ectopic discharges following peripheral axotomy are critically important and are thus the main focus of this proposal. An underlying hypothesis is that, if the mechanisms of impulse generation are understood, they can be better controlled, with a resultant reduction in central sensitization and thus in neuropathic pain. Four specific aims are proposed to unravel the mechanisms of ectopic impulse generation. The first is to show that there is a baseline component of ectopic discharge that is independent of activity in the sympathetic nervous system and an additive component that is dependent on sympathetic activity. This will be shown by in-vivo single dorsal root fiber recordings. The second aim is to show which sodium channel subtypes are up-regulated after nerve lesions, the assumption being that the up-regulated channels are important in impulse generation. This will be done with in-vivo and in-vitro electrophysiological recordings and molecular biological and immunohistochemical techniques. The third aim is to show which subtypes of adrenergic receptors are up-regulated, using the same techniques and assumptions as the second aim. The fourth aim is to determine the roles of purinergic and peptidergic systems in the generation of ectopic discharges. The effects of agonists and antagonists for ATP and neuropeptide Y, given either alone or in conjunction with adrenergic agents, on ectopic discharges in neuropathic rats will be examined using electrophysiological and pharmacological techniques.